An exhaust gas recirculation device for recirculating an exhaust gas into combustion chambers by introducing into an intake passage of an engine, an exhaust gas discharged from the combustion chambers of the engine to an exhaust passage, is disclosed in the specification of US Patent Publication No. 2009/013253. Much inert gas such as carbon dioxide is included in the exhaust gas, and therefore a temperature of combustion of fuel in the combustion chamber is decreased by recirculating the exhaust gas into the combustion chamber. Accordingly, the production of nitrogen oxide along with the combustion of the fuel in the combustion chamber is restricted.
The specification of the above-mentioned US patent Publication discloses the exhaust gas recirculation device comprising an exhaust gas recirculation passage (hereinafter, this passage is referred to as “upstream exhaust gas recirculation passage”) for introducing the exhaust gas from the exhaust passage upstream of a turbine of a turbocharger into the intake passage downstream of a compressor of the turbocharger, and an exhaust gas recirculation passage (hereinafter, this passage is referred to as “downstream exhaust gas recirculation passage”) for introducing the exhaust gas from the exhaust passage downstream of the turbine of the turbocharger into the intake passage upstream of the compressor of the turbocharger.
The specification of the above-mentioned US patent Publication does not clearly disclose a difference in the property between the exhaust gas introduced into the intake passage via the upstream exhaust gas recirculation passage and the exhaust gas introduced into the intake passage via the downstream exhaust gas recirculation passage, however, generally, a pressure and a temperature of the exhaust gas introduced into the intake passage via the upstream exhaust gas recirculation passage are relatively high, while a pressure and a temperature of the exhaust gas introduced into the intake passage via the downstream exhaust gas recirculation passage are relatively low.
In the exhaust gas recirculation device disclosed in the specification of the above-mentioned US patent Publication, in consideration of matters such as an improvement of fuel consumption and a decrease of exhaust emission, on the assumption that the property of the exhaust gas introduced into the intake passage via the upstream exhaust gas recirculation passage is different from that of the exhaust gas introduced into the intake passage via the downstream exhaust gas recirculation passage, the ratio of the amounts of the exhaust gases introduced into the intake passage via the upstream and downstream exhaust gas recirculation passages are controlled.
As explained above, when the temperature of the combustion of the fuel in the combustion chamber becomes low, the production of the nitrogen oxide along with the combustion of the fuel in the combustion chamber is restricted. An exhaust gas recirculation device of an engine is known wherein a temperature in the combustion chamber (hereinafter, this temperature is referred to as “cylinder temperature”) for restricting the amount of the produced nitrogen oxide to an allowable amount is set as a reference cylinder temperature, and the ratio of the amounts of the exhaust gases introduced into the intake passage via the upstream and downstream exhaust gas recirculation passages is controlled such that the cylinder temperature becomes lower than the reference cylinder temperature.
As explained above, the cylinder temperature used for controlling the cylinder temperature to a temperature lower than the reference cylinder temperature in order to restrict the production of the nitrogen oxide along with the combustion of the fuel in the combustion chamber, is a temperature obtained by averaging temperatures in regions of the combustion chamber on the combustion of the fuel. Accordingly, when the temperature in a part of the regions of the combustion chamber on the combustion of the fuel is extremely high and the temperature in the remaining regions is relatively low, it is realized that the cylinder temperature is relatively low. Therefore, even when the cylinder temperature is controlled to a temperature lower than the above-mentioned reference cylinder temperature, the temperatures in a part of the regions of the combustion chamber may be extremely high. By the study of the inventors of this application, it is realized that compared with the cylinder temperature, the temperature in the local region in the combustion chamber largely relates to the production of the nitrogen oxide along with the combustion of the fuel in the combustion chamber. That is, it is realized that even when the cylinder temperature is controlled to a temperature lower than the reference cylinder temperature, the amount of the produced nitrogen oxide along with the combustion of the fuel in the combustion chamber can not be restricted to an allowable amount, when the temperature in a part of the regions of the combustion chamber is extremely high.